Ask the disposal company you work with. Back when I still ran a ATL the local one wanted all stages mixed, except for the fixer/blix (which goes to silver recovery, and in the case of fixer, recycling).

And earlier on ready-mix chemicals were usually disposed of down the drain - where they inevitably mixed.

But read the specs for warnings and exceptions! Some chemicals used outside regular processing (like Farmer's reducer, selenium toners) can be dangerous when mixed with acids...

They don't know.
They are not used anymore to work with photo chemicals.
But at least, giving it to them, I don't have to throw the chemicals into the public sewage, which isn't nice.

I usually mix stop and fixer in the same bottle, in small quantities. Sometimes developer and fixer. Having a bottle per chemical is bulky and not very comfortable. Specially with C41 and RA4. Too many bottles.

I just never mix stop with developer.

I read the manuals thoroughly, but haven't find any info about this. Knowing which chemicals can be mixed together would be of great help, for many people, I suppose, and probably preventing some from just pouring them down the drain.

I usually mix stop and fixer in the same bottle, in small quantities. Sometimes developer and fixer.

Stop (acetic or citric acid) is a component of acidic fixer, so these may be mixed.

Mixing anything with alkaline (colour) fixer or blix will however make it hard to regenerate them and recover the silver (mandatory in Germany), so they advise against that in product literature for pro fixes/blixes. My literature does not mention that it has any other negative consequences (like releasing toxic or smelly fumes, or forming a concrete-like precipitate), but that might simply mean that they never tried.

Some consumer grade small colour kits came with recommendations to mix all chemicals in the order of consumption, so some formulations do mix, but YMMV.

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